Tumor invasion and metastasis is mediated by a transition from an epithelial to a mesenchymal phenotype. E-cadherin downregulation in epithelial tumors is associated with tumor progression. In contrast, N-cadherin, normally not expressed in epithelia, is upregulated in breast tumors and is associated with an invasive phenotype. R-cadherin, another classic cadherin known to function in brain, retina, muscle, pancreas, and kidney, has only very recently been linked to epithelial carcinomas. We found that R-cadherin is highly expressed in the breast epithelium and non-transformed mammary cell lines. In breast cancer, R-cadherin is highly expressed in ductal carcinomas in situ but markedly reduced in invasive duct carcinomas especially in advanced tumor areas, suggesting R-cadherin is lost with tumor progression. In support of a tumor suppressive role, knockdown of R-cadherin in normal breast cells disrupted morphogenesis and stimulated invasiveness, whereas overexpression in invasive breast cancer cells induced morphogenesis and inhibited invasion, tumor formation and lung metastasis. Thus, R-cadherin is an overlooked and important epithelial cadherin which exerts a potent suppressive role in breast cancer. In light of the widely appreciated roles that cadherins play in cancer progression, it will be important to elucidate whether R-cadherin is a critical factor and elucidate whether it acts through an adhesive or signaling mechanism. In Aim 1, we will determine whether R- cad downregulation is associated with malignant progression and evaluate methylation as a mechanism for R- cadherin loss. In Aim 2, we will map the regions within R-cadherin that determine its suppressive function and differentiate it from the proinvasive N-cadherin. Establishing the clinical relevance of R-cadherin and identifying the segments which mediate suppression could provide leads into signaling pathways and therapeutic strategies to control breast cancer. PUBLIC HEALTH RELEVANCE: We found that R-cadherin is substantially downregulated in breast cancer. We have established a series of in vitro and in vivo models to test tumor suppression by R-cadherin. Interestingly, R-cadherin and N- cadherin are structurally very similar, yet display opposite effects on tumor progression. We have designed a series of studies to study the role of R-cadherin loss in breast cancer progression and map the molecular determinants that mediate its suppressive function as opposed to those that mediate tumor invasion by N- cadherin. We believe these studies have the potential to broaden our understanding of mechanisms that regulate breast cancer progression and have thus the potential to provide insights into therapeutic application.